A valve regulated lead-acid battery can be used in any posture since an electrolyte solution is gelated or is held by a porous body such as a retainer mat, and therefore the amount of a free electrolyte solution can be reduced. Further, it is unnecessary to resupply water since oxygen gas generated at a positive electrode during charge can be absorbed and reduced to water at a negative electrode, and therefore water in the electrolyte solution is hardly lost. On the other hand, in the valve regulated lead-acid battery, since the amount of the electrolyte solution is small, the concentration of sulfate ions in the electrolyte solution tends to decrease by discharge. When the concentration of sulfate ions in the electrolyte solution is decreased, the solubility of lead and lead sulfate in the electrode plate becomes high and lead and lead sulfate tend to be eluted in the electrolyte solution in the form of Pb2+. When the lead-acid battery is charged after overdischarge, Pb2+ eluted in the retainer mat is reduced to a dendritic metal lead, and therefore a short circuit may occur between the positive electrode plate and the negative electrode plate. This kind of short circuit is referred to as a penetration short circuit.
The present inventor et al. have proposed to add a silica sol to an electrolyte solution in order to prevent the penetration short circuit in a valve regulated lead-acid battery (Patent Document 1 JP-A-2008-204638). However, even though a silica sol is added singly, the effect of suppressing a short circuit after the valve regulated lead-acid battery is left in an overdischarge state is not sufficient (Table 1), and charge acceptance after the valve regulated lead-acid battery is left in an overdischarge state is not sufficient. Thus, it is necessary to further reduce the short circuit after the valve, regulated lead-acid battery is left in an overdischarge state, and to improve the charge acceptance after the valve regulated lead-acid battery is left in an overdischarge state.
In addition, Patent Document 2 (JP-A-2008-243487) discloses that the utilization factor of a positive electrode is improved when a liquid type lead-acid battery contains lithium ions. However, JP-A-2008-243487 does not describe a relation between the lithium ions and the penetration short circuit or charge acceptance after the lead-acid battery is left in an overdischarge state.